Typical rotating anode X-ray tubes include a beam of electrons directed through a vacuum and across a very high voltage (on the order of 100 kilovolts) from a cathode to a focal spot position on an anode. X-rays are generated as electrons strike the anode, which typically includes a tungsten target track, which is rotated at a high velocity.
The conversion efficiency of X-ray tubes is relatively low, i.e. typically less than 1% of the total power input. The remainder is converted to thermal energy or heat. Accordingly, heat removal, or other effective procedures for managing heat, tends to be a major concern in X-ray tube design.
HV electric power cables are typically used to provide the requisite over 100 kilovolt potential difference between the cathode and anode, in order to generate the aforementioned X-rays. One end of the cable is connected to a power source, and the other end is connected to the tube, for connection to the cathode, by means of an HV connector assembly. The connector assembly generally includes a holding structure for maintaining the end of the cable with respect to the tube, such that the end portion of the cable conductors can be joined to a tube. The cable conductors typically include either a single conductor or a number of conductors.
The connector assembly further includes a quantity of HV insulation surrounding any exposed portion of the cable conductors which lie outside the tube. The HV insulation is joined to the X-ray tube and is relatively thick, in relation to the high voltage of the cable conductors.
Generally, high voltage insulating materials, such as epoxy, also tend to be very poor thermal conductors. This creates undesirable results when an HV connector assembly is directly attached to an X-ray tube, such as across an end thereof.
As stated above, a large quantity of heat is generated in the X-ray tube, as an undesired byproduct of X-ray generation. A portion of this heat is directed against the connector insulation material, which has a comparatively large area contacting the tube. Because of its poor thermal conductive properties, this insulator serves as a heat barrier such that a substantial amount of heat tends to accumulate proximate to the connector. Resultantly, the temperature limits of the connector insulation may be readily exceeded, such that the steady state performance of an X-ray tube is limited.
To improve clinic throughput, X-ray tube designers are facing an ever-increasing demand for more power. Traditionally, CT tubes have included a bi-polar HV system to generate X-ray beams, where a cathode and anode operate at 70 kV under different polarities. A bi-polar HV system typically uses a Federal standard receptacle/plug to bring the HV into the tube casing, where HV connections are made in oil through HV Feedthrough to a tube insert.
HV components within bipolar systems are rated on the order of 70 kv. In an effort to allow more tube peak power, a configuration with mono-polar HV system has been implemented. A mono-polar tube operates at 140 kV with negative polarity and includes a grounded anode electrode.
Mono-polar systems have numerous challenges in terms of HV clearance, discharge activities due to a much higher operating voltage, and constrained dimensions. Conical insulators/plugs have been implemented for such configurations. Several reliability and performance issues have been identified, however, due to thermal stress and material degradation of these conical devices. Conical HV insulation is therefore generally not a viable option for high power tubes.
One of major challenges an HV connector faces is HV integrity under high power conditions. For a continuous high power application, connector temperatures may exceed material limits. Consequently a catastrophic failure may occur through electric breakdown due to thermal runaway or long term discharges from associated material degradation, related to excessive temperatures.
Typical HV solutions often have difficulties handling high temperature scenarios including temperatures in excess of 150° C. Components that include EPR rubber, which is only rated at 105° C. continuously, are of great concern for such applications.
The disadvantages associated with current X-ray systems have made it apparent that a new technique for HV connection to X-ray systems is needed. The new technique should include robust response to thermal stress and should also prevent material degradation, while still maintaining a superior HV performance. The present invention is directed to these ends.